A separation membrane used in separating gas, liquid or solid, particularly a specific component such as an ion material has been designed to have selectivity to a material which is removed and simultaneously allow a material which is permeated to pass with low resistance by appropriately combining a dense structure and a porous structure in order to selectively permeate or eliminate specific components.
Recently, a technology using a separation membrane having the structure has been frequently applied even to a process of purifying water and treating wastewater. These separation membranes are classified into a polymer membrane, a ceramic membrane, a metal membrane, and an organic/inorganic composite membrane according to the material, and are divided into micro-filtration (MF), ultra-filtration (UF), nano-filtration (NF), and reverse osmosis (RO) membranes according to the performance.
Meanwhile, in a method for manufacturing a separation membrane using a polymer resin as a material, a non-solvent induced phase separation method is generally used, in which a polymer solution including a good solvent and a pore-forming agent is cast and extrusion-spun at low temperature, in which phase separation by heat does not occur, to solidify a polymer resin in a non-solvent, and thus form a porous structure. The non-solvent induced phase separation method has an advantage in that the size of pores may be freely adjusted, but there is a disadvantage that when finger-like macrovoids are formed, mechanical strength of a separation membrane is so weak that the membrane is broken during the operation.
As another method, a thermally induced phase separation method is a method for manufacturing a separation membrane by spinning a polymer resin at a temperature in which the phase separation occurs or at a higher temperature, and cooling and solidifying the polymer resin, and it is general to exhibit a spherical structure by a crystal of the polymer, particularly, spherulite. In the thermally induced phase separation method, there is an advantage in that a separation membrane with a strong mechanical strength is easily manufactured, but it is difficult to make the pore size so small as the size of an ultra-filtration membrane.
For the separation membrane by the non-solvent or thermally induced phase separation method, a separation membrane has been manufactured by melting and mixing a polymer resin, a solvent, and an additive using an equipment in the form of a reactor with an agitator; removing bubbles; and then performing extrusion under pressure of nitrogen or a gear pump. However, there are disadvantages that it takes a lot of time to melt the polymer and perform a stabilization process of removing bubbles; it is difficult to extrude a polymer melt having high viscosity; and that if the extruding and spinning is carried out at a high temperature, it is difficult to manage the process, for example the temperature of lines connected to nozzle must be controlled.
Recently, there have been attempts in the industrial fields to manufacture a separation membrane using an extruder such that these disadvantages are alleviated and a continuous process is achieved. However, in order to melt a polymer pellet or powder in advance, the temperature needs to be increased to a melting point of the polymer or higher, and it is difficult to control the thermally induced phase separation during the process of cooling the polymer solution to a room temperature.
Meanwhile, in a separation membrane generally used in water treatment, contaminants are adsorbed and grown on the surface of the membrane while the separation membrane filters the contaminated original water, thereby generating contamination on the surface of the membrane. The aggravation of the separation membrane contamination increases the water permeation pressure and gradually decreases production quantities, thereby ultimately reducing the filtration function of the separation membrane. In order to control contamination of the separation membrane, cleaning is performed using chlorine-based materials and acids and alkalis. However, since this shortens the lifespan of the separation membrane, a polyvinylidene fluoride-based resin, which is a material with high chemical resistance, has recently been used, or studies on hydrophilizing the separation membrane have been conducted in order to reduce contamination of a hydrophobic material such as protein.
A polyvinylidene fluoride (PVDF) separation membrane manufactured by a typical process is vulnerable to contamination because its surface is hydrophobic. As a general hydrophilization method to prevent this problem, a surface modification or coating method has often been used in the post-treatment process. However, in this case, there is a problem in uniformity and durability of the surface.
Further, there is a method of blending hydrophilic polymers when preparing a polymer solution. In this case, the productivity is good, but most of the hydrophilic polymers have bad compatibility with polyvinylidene fluoride, and thus it is difficult to have a uniform distribution in a micro scale within the separation membrane. In addition, lots of the hydrophilic polymers are dissolved out from the membrane in the process of washing/extracting with water or other solvents for removing excessive solvent after the phase separation. Furthermore, even if the hydrophilic polymers are remained in the final membrane, they are dissolved out from the membrane into water over a long period of time of using, thereby causing harms to the human body.
In this regard, Korean Patent Application Laid-Open 10-2008-0033279 describes a method of cross-linking hydrophilic components to polyvinylidene fluorides by manufacturing a membrane from a polymer blend containing hydrophilic components such as PVP and heating and/or applying radiation to the membrane manufactured in order to solve the problem that the hydrophilic components are easily dissolved out to water and improve the hydrophilic stability.
However, this method cross-links the hydrophilic components after the hollow fiber membrane is completely manufactured, and thus still has a problem that the hydrophilic polymers having bad compatibility cannot uniformly distributed in a micro-scale within the polyvinylidene fluoride separation membrane, and that the cross-linking reaction also occurs in a limited manner. Furthermore, in the method, a subsequent process of cross-linking the hydrophilic components is required after the completion of the membrane synthesis after wetting so that the productivity is low and it takes a lot of time to carry out the method. Therefore, there is need for consideration of a novel method which may maintain durability along with uniformity and achieve a continuous process for a high productivity.
Throughout the present specification, a plurality of papers and patent documents are referenced, and citations thereof are indicated. The disclosure of each of the cited papers and patent documents is incorporated herein by reference in its entirety to describe the level of the technical field to which the present invention pertains and the content of the present invention more apparently.